1. Field of the Invention
This invention relates to memory integrated-circuits and, more particularly, to a circuit for operating the memory integrated-circuit in a test mode.
2. Prior Art
A memory integrated-circuit is placed into a test mode so that various parameters of the memory integrated-circuit can be monitored. A number of different performance parameters can be monitored and can include anything that a design engineer thinks is important and needs to be monitored.
One way to put a memory integrated-circuit into a test mode is to provide a forbidden or don't-care combination of conventional input signal levels at various input terminals. This forbidden or don't-care combination of conventional input signal levels is then detected inside the integrated-circuit and the integrated-circuit goes into test mode. A difficulty is that some memory chips do not have a forbidden or don't-care combination of conventional input signal levels.
One example of a manufacturer being unable to use a don't-care package pin for detection of a test mode command for a 1 Meg RAM integrated-circuit is that a customer could create his test board to use the trace and socket associated with this don't-care pin for another purpose in order to save circuit-board space so that the don't-care pin cannot be used for detection of a test mode.
Another example of a manufacturer being unable to use a don't-care package pin for detection of a test mode command for an integrated-circuit is that, when a RAM is being written to, the output enable (OE) pin is in a don't-care state. This is because the write condition automatically shuts off the outputs which is normally a function of the OE signal. On the face of it, it would seem that the OE being high and the write condition being active would be a good state to use for test mode detection. But an astute customer could take advantage of OE being a don't-care during the RAM write condition to use the OE trace on the circuit board for another purpose. This would preclude this avenue for test mode detection.
These two examples indicate that there are cases where there is no combination of conventional signal inputs which can be detected to put an integrated circuit into test mode.
Consequently, a need exists for a circuit which is able to get around this limitation and to be able to put an integrated-circuit into test mode even though there is no use of forbidden or don't-care combinations of conventional input signal levels.